1. Field of the Invention
The present invention relates to the art of a structure of a power transmission unit for automobiles.
2. Discussion of the Related Art
JP-A-8-295140 describes an example of the power transmission unit of a hybrid vehicle. The hybrid vehicle taught by JP-A-8-295140 is comprised of an engine, a generator/motor for generating electric power from input rotation, an electric motor driven by an electric current supplied thereto, a planetary gear unit, and a braking means. In the hybrid vehicle, an output shaft is connected to the electric motor to be rotated by the electric motor, a sun gear of the planetary gear unit is connected to the generator/motor, and a carrier of the planetary gear unit is connected to the engine. The braking means is adapted to stop rotations of the carrier and the engine.
According to the teachings of JP-A-8-295140, the engine is stopped and the electric motor is driven if a driving force required for driving the hybrid vehicle is larger than a set value. In this case, the braking means is actuated and the generator/motor is operated as a motor. Specifically, a one-way clutch is employed as the braking means, and the one-way clutch is disposed between an output shaft of the engine and a casing of a transmission holding the motors and the planetary gear unit. The one-way clutch becomes free when the engine rotates in its normal direction, and locks when the engine rotates in a reverse direction. Alternatively, a brake may also be used as the braking means in the hybrid vehicle taught by JP-A-8-295140 instead of the one-way clutch.
Thus, in the hybrid vehicle taught by JP-A-8-295140, the one-way clutch or the brake is disposed between the engine block and the casing of the transmission to serve as the braking means for locking a rotation of the engine. Basically, a clearance between the engine block and the housing of the transmission is not especially sealed. According to the conventional art, therefore, a through hole is formed on a lower portion of the housing of the transmission to drain water entering into the housing.
However, the through hole of the housing also allows entrance of water and dusts into the housing. Such foreign matter may further intrude into the braking means, and adhesion of the foreign matter to a movable portion of the braking means may cause a malfunction of the braking means and deterioration in durability of the braking means.
The present invention has been conceived noting the foregoing technical problems, and it is therefore an object of the present invention is to provide a power transmission unit for preventing the ingress of foreign matters into a locking device such as a one-way clutch and a brake in a housing, thereby preventing a malfunction of the locking device and deterioration in durability of the locking device.